The idea of training horses in a humane way while studying and mimicking equine body language was largely popularized by horseman and author, Monty Roberts.
However, Monty Roberts is certainly not the first person to wear the title of 'Horse Whisperer'; there is a long history of training horses without the use of cruelty and brute force. One such person was John Solomon Rarey whose techniques for taming abused and dangerous horses made him famous in the 19th century. His method really took off when he came to work with one of Queen Victoria's horses.
Some of his training techniques have been depicted in the film The Horse Whisperer. He also wrote a book entitled The Complete Horse Tamer, which much of it can be viewed here.
Going back even further, horsemen such as farriers and farmers would use more arcane methods to gentle horses.
From areas in England like Cornwall and East Anglia, as well as Scotland, there are stories of folks using witchcraft to tame and train horses, and there were even secret societies established for these horse whisperers.
One such group is The Society for the Horseman's Word, which reputedly began in the 18th century and ended in the 1930's in Scotland (although some suspect that it still may be practised in remote places in Scotland). The Society eventually made it's way down to England where the Society of Horsemen was formed.
To become a member of the Society, a horseman was invited by an established member, and the calling card was a single horsehair. He would go through an initiation ceremony and ordeals, and take a series of oaths; in return he would learn the guarded knowledge of the Society.
The actual 'Horseman's Word' was passed to members after initiation, and it probably differed from region to region. The Word was used to tame and bewitch the horse.
There were also the practitioners called the Toadmen in East Anglia, whose practices also spread out to other parts of England, like Cornwall, as well as to Wales.
The Toadman was initiated through a rather gruesome ritual that involved killing and skinning a toad (less often a frog), and retrieving a V-shaped bone (perhaps the pelvic bone) from the animal. To see details of the actual ritual, click here and here.
The bone was a charm used to 'jade' or bring the horse to a stop. Another charm was made from milt to 'draw' or attract a horse. The milt is a substance found in a foal's mouth right after birth, that looks something like liver. It was carefully removed from the foal's mouth and dried to be worn by the horseman.
There were other substances used to draw and jade a horse; oils were often smeared on the bodies of the horsemen such as a mixture of oil of origanum, oil of rosemary, oil of cinnamon, and oil of fennel (source here) to draw. Substances such as stoat's liver and rabbit's liver, dried and powdered up with dragon's blood/palm resin (source here).
One may not be keen on turning to the esoteric for their horse training needs, but there is certainly enough literature out there now, as well as courses and trainers, that there is no reason to 'break' a horse to train them.
The equine are wonderful creatures and make excellent co-workers and companions, and I think it is high time that people stop practicing barbaric and cruel training methods, and go for the kinder and gentler (and in the long run more effective!) route.